Plugin architecture (#4861)

* wip

* Refactor, tasks, processors, routes loading

* Move Slack settings config to plugin

* Fix translations in plugins

* Move Slack auth to plugin

* test

* Move other slack-related files into plugin

* Forgot to save

* refactor

app/utils/plugins.ts

@@ -0,0 +1,37 @@
+interface Plugin {
+  id: string;
+  config: {
+    name: string;
+    description: string;
+    requiredEnvVars?: string[];
+  };
+  settings: React.FC;
+  icon: React.FC;
+}
+
+export function loadPlugins(): { [id: string]: Plugin } {
+  const plugins = {};
+
+  function importAll(r: any, property: string) {
+    r.keys().forEach((key: string) => {
+      const id = key.split("/")[1];
+      plugins[id] = plugins[id] || {
+        id,
+      };
+
+      const plugin = r(key);
+      plugins[id][property] = "default" in plugin ? plugin.default : plugin;
+    });
+  }
+  importAll(
+    require.context("../../plugins", true, /client\/Settings\.[tj]sx?$/),
+    "settings"
+  );
+  importAll(
+    require.context("../../plugins", true, /client\/Icon\.[tj]sx?$/),
+    "icon"
+  );
+  importAll(require.context("../../plugins", true, /plugin\.json?$/), "config");
+
+  return plugins;
+}